You can start firm/Company as per your choice. Firm registration is easier and less expensive.
only thing you need to draft partnership deed in stamp paper. (In kerala it is Rs 1000/-stamp paper required)
value of stamp paper may be different in other places. Minimum two person is required to form a Company or a Partnership firm. Both must have attained majority and of sound mind and capable of entering into a valid contact. and apply in form no I attested by advocate/CA/CS/ICWA only. and pay the required fee.
within two weeks you will receive registration certificate. after getting registration certificate you can open bank account and apply for PAN in the name of the firm. Partnership registration is optional. but it is advisable to register it.
for the day to day operation you can use tally accounting package or similar one.